For this trophy / achievement you must find a tiny black bird called "Bristly Crake". It rides on the back of Mosswines in Rotten Vale (and only in Rotten Vale, not on other maps). It can spawn at any daytime and on any rank (low-rank & high-rank).
It is an ultra rare spawn. However, there are 2 locations you can check in one swoop. See the video for the exact locations and what the creature looks like. Check them both in one visit and if the creature is not in either spot, fast travel to another map (such as Astera or Wildspire Waste) and return. You have to travel to another map to get a new try! Fast traveling between camps in Rotten Vale won't do anything.

Important: Leave your Palico behind (talk to your cat at camp and tell it to stay) + equip the Ghillie Mantle. This way you won't scare off the rare bird. If your Palico is with you it could scare the bird. The Ghillie mantle renders you invisible to animals so they won't even know you're there. Get close and use the capture net to make this ultra rare catch.